Attempts to combine fixed and fluid-bed catalytic reactions in one bed have been made previously. U.S. Pat. No. 3,374,052 discloses a vessel with a movable top plate that closes in on the top of the fluid bed. This pressure from the plate renders the top of the bed non-fluidized, or fixed. This method, however, would be difficult to apply to a chemical reaction involving more than one type of catalyst in the same reactor, and this fixed-bed section is not interspersed throughout the fluid catalyst bed, resulting in disadvantages such as heat transfer capabilities.
The fact that catalysts may be of fixed or fluid type is known in the art.
The selection of the type of bed for a given reaction is dependent on several factors. Fixed-bed reactors tend to provide excellent gas solids contacting, less catalyst than for a fluid bed, and a smaller reactor. Fixed-bed reactors also allow the grading of catalyst within the reactor, and different types of catalyst may be installed at different locations. However, in reactions in which there is a large amount of heat released or absorbed, fixed-bed reactors require a large amount of heat transfer surface to adequately control the reaction temperature. The fixed-bed particles are normally much larger than fluid-bed particles, and therefore are more sensitive to reactant diffusion complications. Finally, fixed-bed reactors generally require efficient mixing of all reactants prior to entering the catalyst bed. The type of reaction to be employed in a particular operation can be determined only after considering all these features.
Fluid-bed reactors are achieved where gas is passed through the reactor at a sufficient velocity to "fluidize" the catalyst particles. Because of this fluidized state, such beds have excellent heat conductivity and require only moderate amounts of heat transfer area. The feeds do not have to be well mixed, for this may take place within the bed. However, along with increased reactor size, a fluid bed tends to bypass reactant gas due to bubble formation. Grading of the catalyst is normally not possible due to the high degree of mixing in such a bed.
The present invention solves the problems normally associated with each type of bed, yet retains the advantages of both. In reactions involving several stages or steps, the present invention allows the combination of such steps in one reactor.